1. Field of the Invention
The present invention relates to a method, system, and article of manufacture for self-servowriting a disk.
2. Description of the Related Art
Disk drive units with stacked, platter-shaped rigid magnetic disks are used for data storage. The platter-shaped disks rotate about a drive axis. A disk drive unit uses actuators to position a plurality of transducer heads, such as drive heads, that move radially to the drive axis to write data to the surfaces of the disks and read data from the surfaces of the disks. Disks store data in concentric tracks and, in the current art, the density of the tracks may exceed 100,000 tracks per inch.
To read and write data, a magnetic head must remain accurately centered on a selected track. At high track densities, the head may have to stay centered on the narrow tracks to within a tolerance better than a millionth of an inch. To achieve this level of precision, the head reads position information from tracks permanently written onto a disk surface. The position information is written in special codes called servo codes. A linear feedback system continually uses the position information (servo codes) to adjust the head to correct for position errors.
In prior art, during manufacture of the disk, a servowriter unit writes the servo codes on a disk, and the position information remains on the disk for the life of the disk. The servowriter is a specialized device used in the manufacture of the disk that includes a large base to minimize the effects of vibration, precision fixtures to hold the target disk drive, and a precision laser interferometer based actuator arm positioning mechanism to precisely position the arms radially with respect to the axis of rotation of the disks in the drive. With high servo track densities, the mechanical vibrations of the disk relative to the external sensors can affect the accuracy of the servo writer systems. In addition, a clean room environment is required by servo writer systems. Furthermore, servo writer systems are difficult to use to write servo codes on small disk drives.
Instead of using a servo writing system, self-servo writing techniques may be used. In self-servo writing, the disk head writes servo patterns as well as reads servo patterns. The servo codes have to be written with a high degree of accuracy in self-servo writing in high-density disk drivers.
FIG. 1 illustrates a schematic diagram of servo tracks on a disk during self-servo writing in a manner known in the prior art. In prior art, a head 20 can read and write servo bursts A, B, C, D (reference numerals 22, 24, 26, 28 respectively) on the disk. The servo bursts may be any pattern, whose signal amplitude can be read by the head 20. The servo bursts are of the same width as the width of a data track. The centerlines of the servo bursts form servo tracks on a disk. Since, each servo burst pattern is offset a half servo track width from the previous servo burst pattern, there are twice as many servo tracks as data tracks on a disk. To write the servo bursts, the head 20 would first write servo burst A 22. After writing servo burst A 22, the head 20 moves to a position where the head reads half of the servo burst A 22, and nothing (i.e., the blank disk). The data read by the head 20 is expressed as a combination of the amplitude of half of servo burst A 30 (obtained by reading half of the servo burst A 22) and white noise 32 (obtained by reading nothing). The head writes the servo burst B 24. However, because of the white noise 32 the positioning of the head 20 may not be exactly at the half servo track offset from servo burst A 22 while writing servo burst B 24. Head 20 writes servo burst C 26 after moving to a position where the head 20 reads half of servo burst B 24, and nothing. Subsequently, head 20 writes servo burst D 28 after moving to a position where the head 20 reads half of servo burst C and nothing.
The head 20 can self-servo write an entire disk surface. The head 20 starts adjacent to a crash stop on the disk surface (the crash stop is a fixed position on the disk at or around the center of the disk adjacent to which is the first servo track). The head 20 writes a servo burst on the first servo track. The head 20 then moves to a position where the head 20 reads half of the first servo track and nothing. The head 20 then writes a servo burst on the second servo track. In such a manner the head 20 writes servo tracks from servo track one through servo track n. At servo track n the head 20 moves to a new position where the head 20 reads an amplitude equal to half of the servo burst for servo track n and nothing. The head 20 writes the servo burst for servo track n+1 at the new position. In such a manner the head 20 writes servo bursts on servo tracks till the periphery of the disk. However, the servo patterns have positional errors on the disk because white noise 32 prevents the head 20 from being positioned at a half servo track width offset from the previous servo burst pattern.
FIG. 2 illustrates a prior art self-servo technique used when servo tracks are written to a disk with multiple surfaces or multiple disks. A first head 48a track follows, i.e. the first head 48a reads servo burst A 49 from a first surface 50a and positions the first head 48a such that a second head 48b, and a third head 48c can write to disk surfaces on the basis of the position of the first head 48a. 
The second head 48b and a third head 48c write servo bursts B 51, 52 to second surface 48b and third surface 48c respectively, on the basis of the positioning of the first head 48a FIG. 2 shows that track following at 50% of the amplitude of the servo burst A 49 by the first head 48a is imprecise because of the presence of white noise (white noise is the signal read from the blank region of a disk surface). If no white noise or error introducing factors are present, then when the first head 48a reads 50% of the amplitude of servo burst A 49, the first head 48a would be half way across the servo burst A 49. Since on physical disk surfaces the head 48a reads white noise in addition to the amplitude of servo burst A 49, the first head 48a cannot be positioned exactly half way across the servo burst A 49. In particular, when a disk has a very high number of tracks per inch (for example, over 100,000 tracks per inch), track following is extremely difficult because the actuator movements cannot position the head 48a to the precision necessary for track following at 50%.
FIG. 3 illustrates a prior art technique for positioning the head more precisely than the prior art techniques shown in FIGS. 1 and 2. The figure shows a head 64 and the centerlines of servo track n−1 65a, servo track n 65b, and servo track n+1 65c. FIG. 3 also shows a servo burst A 66 centered on servo track (n−1) 65a, and a servo burst C 68 centered on servo track n+1 65c. The head 64 moves to a new track position over the servo burst A 66, where the head 64 reads half of servo burst A 70 and white noise 72. At the new position, the head 64 also reads half of servo burst C 74 and white noise 76. By subtracting the half of servo burst C 74 and the white noise 76 from the half of servo burst A 70 and the white noise 72, the head 64 eliminates the effect of the white noise 72 and 76, and the head 64 determines the position that is at a half servo track width offset from servo burst A more precisely when compared to FIGS. 1 and 2. In FIG. 3, the head 64 track follows more precisely when compared to situations where the effect of white noise is not eliminated. With FIG. 3, because servo bursts on servo track n−1 65a and servo track n+1 65c are known, the head 64 can be positioned with greater precision on servo track n 65b by the noise elimination technique outlined above. As a result higher track densities can be supported in a disk. Such a noise elimination technique is known in prior art.
The prior art self-servo writing techniques uses the read sensors to read servo information from the previously written tracks. The sensor is positioned on the half track boundary and by design the sensor reads half servo signal information and half no signal information, where the no signal information equates to typically white noise. Reading this white noise introduces a random positioning error into the positioning system. Increasing the tracks per inch may introduce an even greater extent of random unwanted vibration, system electrical noise, disk electrical noise, and read sensor noise, which results in degradation of the servo written information. Servo written information that is not written on the concentric track center exposes the servo track following system in the disk drive to errors and compromises data integrity. Although, the noise cancellation technique described in FIG. 3 is known in prior art, such prior art noise cancellation techniques have disadvantages. For good noise cancellation precise back and forth independent movements of heads may be necessary, and such movements have not been achieved in prior art noise cancellation techniques. These are some of the disadvantages with the prior art solutions. Hence, there is a need in the art to provide for a method, system, and article of manufacture that improves the quality of the servo written tracks.